CS 261 - Winter 2011

Abstract Data Types

Container Classes

• Over the years, programmers have identified a small number of different ways of organizing collections of data.

• These container abstractions are now the fundamental heart of the study of data structures.

• Examples, Stack, Queue, Set, Map, etc

Three Levels of Abstraction

• There are at least three levels of abstraction in the study of data structures

• ADT - Abstract Data Type, language independent

• Interface - the interface in a particular library of containers

• Implementation - the various implementations in a particular library

The ADT view(Abstract Data Type)

• Every data type can be described in a way that is independent of language/library

• E.G., A Stack is a collection that has the property that an item removed is the most recently entered item.

• Properties that are true regardless of the names given to operations in library

Metaphors

• ADT view are often described by metaphors (e.g., stack of plates). Easy to remember. Easy to understand.

The Interface View

• Gives specific names to operationsstruct stack;void initStack (struct stack * stk);void pushStack (struct stack * stk, double val);

double topStack (struct stack * stk);void popStack (struct stack * stk);int isEmptyStack (struct stack * stk);

Additional Information

• The interface view gives only signatures

• Must also attach meanings (LIFO properties of stack, etc)

• Can also attach expected execution times (want push and pop to be constant time).

• Both more and less informative than ADT view

The Implementation View

void pushStack (struct stack * stk, double val) {

stk->data.add(val);}

int stackIsEmpty (struct stack * stk) {

return dyArraySize(stk->data) == 0:}

The Study of Data Structures

• As we study data structures we move constantly between all three views

• The ADT view is constant from one language to the next

• The interface view allows you to compare implementations

• The implementation allows you to see how it is done

The Classic ADTs

• Bag, Ordered Bag - simple collections

• Stack, Queue, Deque - ordered by insertion

• Set - unique elements, fast test• Map (Dictionary) - key/value associations

• Priority Queue - ordered by importance

BAG as Design Pattern

• Problem: Need to maintain an unordered collection of elements, without needing to know how it is organized

• Forces: Require insertion, test and removal - time of insertion is unimportant

• Counter-forces: Time of insertion IS important

• Solution: Provide abstract interface

BAG interface

• Provide functions for operations, effectively hiding implementation (names may vary)

addBag (container, value)testBag (container, value)removeBag (container, value)sizeBag (container, value)

Stack as Design Pattern

• Problem: Maintain collection in Last-In, First-Out format

• Forces: LIFO• Counter-forces: non-LIFO• Solution: again, abstract Stack interface

The Classic Implementation

Techniques• Arrays and Dynamic Arrays (Vectors)

• Linked Lists• Binary Trees, Balanced Trees• Heaps• Hash Tables• Skip Lists• Etc etc etc

Now, your first worksheet

• As a way to introduce C and the worksheet idea, we will make implementations of a simple BAG and STACK using an array

• We will do these together, in class - won’t always be that way

• Worksheets are handed in, and are lightly graded.

First look at C - interface file

# define EleType double# define EQ(a, b) (a == b)

struct arrayBag { EleType data[100]; int count;};

void initBag (struct arrayBag * b); … etc

First function - initialize

void initBag (struct arrayBag * b)

{

}

Add to bag

void addBag (struct arrayBag * b, EleType v)

{

}

Test for contains

int testBag (struct arrayBag * b, EleType v)

{

}

Remove from bag

void removeBag (struct arrayBag * b, EleType v)

{

}

Return size of collection

int bagSize (struct arrayBag * b)

{

}

How about stack?

void pushStack (struct arrayBag * b, EleType v)

{

}

Test for stack empty

int isStackEmpty (struct arrayBag * b)

{

}

Top of stack

EleType topStack (struct arrayBag * b)

{

}

Pop top element

void popStack (struct arrayBag * b)

{

}

That’s all for today

• Hand in your worksheets, put your name on them